In the farm implements art, most farm tractors have separate left rear and right rear brake pedals, which respectively activate the left rear brake or the right rear brake separately of one another. For example, to make a very tight right turn (i.e., to reduce the turn radius of the tractor for a right turn), manual operation of the right rear brake pedal by the operator activates the right rear brake whereby the rotational speed of the braked right rear wheel is reduced, thereby reducing the turn radius of the braked right rear wheel by which the turn radius of the tractor is reduced, such that the tractor almost pivots about the right rear wheel.
In the automotive art, modern dual-circuit hydraulic braking systems for automotive applications typically include an operator-actuated brake actuation unit, such as a tandem master cylinder actuated by a booster-aided brake pedal, by which to supply a first pressurized fluid to each of a first pair of wheel brakes via a first or “primary” braking circuit, and a second pressurized fluid to each of a second pair of wheel brakes via a second or “secondary” braking circuit. The use of wholly redundant braking circuits for operating discrete pairs of wheel brakes ensures continued vehicle braking capability, notwithstanding a degradation of performance of one of the braking circuits. Alternatively, electric actuation of individual wheel brakes is possible as well by techniques well known in the art.
In order to achieve an “anti-lock” brake system (ABS), each braking circuit often features a normally-open electrically-operated inlet valve controlling the flow of pressurized fluid to each wheel brake, while a pressure relief line that includes a normally-closed electrically-operated outlet valve, a return pump, and a check valve controls the return of pressurized fluid from the wheel brake to the brake line upstream of the inlet valve. A “separation” or “isolation” valve, located in the brake line of each circuit upstream of the location at which the pressure relief line connects to the brake line, serves to isolate the brake line from the master cylinder during anti-lock operation.
Increasingly, such anti-lock brake systems are used in combination with wheel speed sensors in a traction control mode. The further addition of a steering angle sensor, a vehicle yaw rate sensor, and a lateral vehicle acceleration sensor in conjunction with vehicle speed, wheel speed, and wheel longitudinal slip enables such anti-lock brake systems to operate in an “electronic stability control” mode, wherein a braking system controller selectively energizes each circuit's electrically-operated valves when the controller identifies an opportunity to enhance vehicle stability through a selective application of the vehicle's brakes. Alternatively, a braking system controller may selectively energize individual wheel brakes through electric actuation.
In order to control the fluid pressure in traction control or vehicle stability control modes, a hydraulic pump is typically placed in the pressure relief line of each circuit downstream of the outlet valve to return pressurized fluid to the circuit's brake line. The pump also serves to provide an increasing rate of fluid pressure upon the closing of the isolation valve to provide a sufficient braking system response time when operating in a traction control mode, even at a time when the brake fluid has a relatively-high viscosity due, for example, to low brake fluid temperatures.
The prior art has recognized, however, that a quicker system response is desirable when the braking system is operated in a vehicle stability control mode. By way of example, a rapid pressure build up in one or the other braking circuit is particularly desirable upon commencing vehicle stability control in order to correct oversteer or understeer conditions. Accordingly, the prior art teaches the addition of a braking circuit pre-charging function to the brake actuation unit, i.e., to the vacuum booster of the master cylinder, in order to increase system response at the time such vehicle stability control is commenced. Alternatively, an additional pre-charging pump is provided in one or both braking circuits to ensure a sufficient increasing rate of fluid pressure at the commencement of vehicle stability control enhancement.
There are multiple Electronic Stability Control (ESC) system implementations on the road today. Although all of them attempt to perform the same task of helping the driver retain reasonable directional control under nonlinear vehicle dynamic conditions, these ESC systems have some distinct implementation differences and can be divided into four categories as defined and described in The Society of Automotive Engineers (SAE) Surface Vehicle Information Report, SAE J2564, “Automotive Stability Enhancement Systems”, revised June, 2004 and superceding version issued December, 2000, which report is hereby incorporated herein by reference in its entirety.
A system is defined as an ESC system in the above referenced report SAE J2564 if it:
a) is computer controlled and the computer contains a closed-loop algorithm designed to limit understeer and oversteer of the vehicle;
b) has a means to determine vehicle yaw velocity and side slip;
c) has a means to monitor driver steering input;
d) has a means of applying and adjusting the vehicle brakes to induce correcting yaw torques to the vehicle; and
e) is operational over the full speed range of the vehicle (except below a low-speed threshold where loss of control is unlikely).
Electronic Stability Control systems in use today can be divided into four categories, as follows.
Type A, comprised of two brake force channels used for yaw stability control (YSC) and three brake force channels used for ABS. Three speed sensors are used, one for each front wheel and one for detecting the average of the two rear wheels.
Type B, comprised of two brake force channels for YSC and traction control, four brake force channels for ABS. Four wheel speed sensors are used at each of the four corners (wheels).
Type C, comprised of four brake force channels for ABS, YSC and traction control. Four wheel speed sensors are used at each of the four corners.
Type D, comprised of a type C system with integrated preemptive control strategies and additional control channels that interface to other than the brake subsystem. These subsystems include, but are not limited to active driveline couplings, and active dampers and stabilizer bars and active steering.
For the vast majority of ESC systems, the corrective yaw moments that are developed by generating tire slip using the vehicle's brake corners are typically hydraulically actuated, but may also use electric actuators to generate the required corner brake force by techniques well known in the art.
Elements that all of these ESC systems have in common include ABS and the ability to sense steering wheel position; the ability to calculate vehicle speed; the ability to sense yaw velocity and lateral acceleration; and the ability to build and control braking force in the channels used for yaw stability control independent of the driver's input to the vehicle braking system. An example of the implementation of a vehicle hydraulic braking system utilizing a Type C or Type D ESC system is described in U.S. Pat. No. 6,896,338, which patent is hereby incorporated herein by reference in its entirety.
Returning now to the concept of minimizing turning radii, it is desirable to have minimization of the turning radius of a motor vehicle. Rear wheel steering, incorporated in vehicles with four wheel steering, can provide a small turning radius; however, four wheel steering is costly and requires a large packaging space around the rear wheels.
Accordingly, what is needed in the prior art is a method of automatically reducing the turn radius of motor vehicles which somehow mimics a farm tractor's ability to have a small turn radius via independently applying the brake of the wheel inside the turn radius by somehow adapting this model to an automotive ESC system.